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Article

Retention of Fine Woody Debris Reduces Stability of Soil Organic Carbon Pool by Changing Soil Organic Carbon Fractions and Enzyme Activities in Urban Picea koraiensis Plantations

by
Honglin Xing
1,†,
Hao Zhang
1,† and
Ling Yang
1,2,*
1
State Key Laboratory of Tree Genetics and Breeding, Northeast Forestry University, Harbin 150040, China
2
College of Forestry, Beijing Forestry University, Beijing 100091, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Forests 2025, 16(3), 434; https://doi.org/10.3390/f16030434
Submission received: 4 February 2025 / Revised: 17 February 2025 / Accepted: 25 February 2025 / Published: 27 February 2025
(This article belongs to the Special Issue Carbon, Nitrogen, and Phosphorus Storage and Cycling in Forest Soil)
Browse Figures
Figure 1
<p>Location of the Forestry Demonstration Base and <span class="html-italic">P. koraiensis</span> plantations (<b>a</b>) and layout of the randomized block design with four FWD retention treatments (<b>b</b>).</p> "> Figure 2
<p>Dissolved organic carbon (<b>a</b>), microbial carbon concentration (<b>b</b>), easily oxidizable organic carbon concentration (<b>c</b>), and particulate organic carbon (<b>d</b>) in urban <span class="html-italic">P. koraiensis</span> plantations with retained FWD. Note: Data are presented as mean ± standard error (<span class="html-italic">n</span> = 3). Different letters mean that there are significant differences between different treatments in the same soil layer (<span class="html-italic">p</span> &lt; 0.05).</p> "> Figure 3
<p>Cellobiohydrolases enzyme activity (CBHs, (<b>a</b>)), β-1,4-glucosidases enzyme activity (βGs, (<b>b</b>)), β-xylosidase enzyme activity (βX, (<b>c</b>)), leucine aminopeptidase enzyme activity (LAP, (<b>d</b>)), and polyphenol oxidase enzyme activity (PPO, (<b>e</b>)) in urban <span class="html-italic">P. koraiensis</span> plantations with retained FWD. Note: Data are presented as mean ± standard error (<span class="html-italic">n</span> = 3). Different letters mean that there are significant differences between different treatments in the same soil layer (<span class="html-italic">p</span> &lt; 0.05).</p> "> Figure 4
<p>The stability of soil organic carbon pool (<b>a</b>) and random forest analysis of environmental factors affecting the stability of soil organic carbon pool (<b>b</b>) in urban <span class="html-italic">P. koraiensis</span> plantations with retained FWD. Note: Data are presented as mean ± standard error (<span class="html-italic">n</span> = 3). Different letters mean that there are significant differences between different treatments in the same soil layer (<span class="html-italic">p</span> &lt; 0.05). The asterisk (*) indicates statistical significance (* <span class="html-italic">p</span> &lt; 0.05).</p> "> Figure 5
<p>Mantel test between soil enzyme and soil factors, as well as Pearson correlation coefficients within soil variables in urban <span class="html-italic">P. koraiensis</span> plantations with retained FWD. Note: The asterisk (*) indicates statistical significance (* <span class="html-italic">p</span> &lt; 0.05).</p> ">
Versions Notes

Abstract

:
The importance of urban forest management and carbon cycle research has increased amidst ongoing urbanization. Understanding the potential impact of fine woody debris (FWD) retention as a management strategy on the soil organic carbon (SOC) levels and stability in urban forests is crucial. In this study, four FWD retention treatments (no retention, CK; low retention, LR; medium retention, MR; and high retention, HR) were implemented in Harbin urban Picea koraiensis Nakai plantations to investigate the stability of the SOC pool in response to these treatments. The FWD retention treatment had no significant effect on the soil’s physical and chemical properties and SOC concentration, but significantly reduced the total potassium and NO3− concentrations. The FWD retention treatment increased active SOC fractions and carbon-degrading enzyme activities, while reducing leucine aminopeptidase, polyphenol oxidase enzyme activities, and the stability of the SOC pool. The random forest model showed that FWD retention, particulate organic carbon, cellobiohydrolases, and β-xylosidase enzyme activities were factors that significantly affected the stability of the SOC pool. These findings suggest that retaining a large amount of FWD in northeast China can benefit the soil carbon cycle in urban plantations by accelerating the turnover of active SOC fractions.

1. Introduction

Urban forests provide a wealth of ecosystem services and sequester significant amounts of carbon as they grow [1]. Many studies have investigated carbon stock and sequestration in urban forests, neglecting the crucial role of woody debris (WD) in urban carbon stock [2]. In forest ecosystems, WD plays a crucial role as a structural and functional component, and can be categorized into fine woody debris (FWD) with a diameter of less than 10 cm and coarse woody debris (CWD) with a diameter of more than 10 cm [3]. Natural or anthropogenic disturbances can alter the fate of carbon in WD, resulting in either stable sequestration in the soil or rapid transfer from the terrestrial biosphere to the atmosphere [4]. To better understand and accurately predict the dynamics of the forest carbon cycle, it is, therefore, necessary to investigate the effects of WD on soil organic carbon (SOC) stocks [5].
Research on the impact of WD on SOC storage has produced divergent results, with some studies reporting increases [4], decreases [6], and no significant effects [7]. Compared to SOC, active SOC fractions (such as microbial carbon (MBC), easily oxidizable organic carbon (EOC), dissolved organic carbon (DOC), and particulate organic carbon (POC)) are susceptible to external influences, have a faster cycle rate, and have a higher validity, so knowledge of active SOC fractions is of greater importance for assessing the SOC cycle and its stability [8]. Peng et al. studied the stability of SOC in a karst faulted basin in Yunnan, China, and suggested that the ratio of POC to SOC in soil can reflect the stability of the soil carbon pool to some extent [9].
SOC storage is governed by microbial mineralization and the sequestration of organic residues. WD may enhance SOC by sequestrating exogenous organic carbon, but it may also accelerate SOC decomposition by boosting microbial activity and soil respiration [10]. Soil enzymes, which are secreted by microorganisms, play a key role in this process, regulating organic carbon degradation and affecting organic carbon dynamics [11]. However, WD is complex organic matter, and different tree species and different amounts of WD have different effects on soil enzyme activities [7]. For example, Noll et al. [12] found that ligninolytic oxidoreductases exhibit a high variability in the WD of deciduous trees. Although these enzymes play an important role in organic carbon turnover, research on how WD affects the activity of carbon- and nitrogen-degrading enzymes remains limited.
Until now, the focus of research has been on analyzing the nutrient content of leachate from CWD or its impact on the SOC pool [6], but the impact of FWD on the SOC pool has often been overlooked [13,14]. Although FWD contributes less carbon than CWD, its faster decomposition may have a more immediate impact on soil properties and carbon dynamics [15]. Studies have also found that additions of different amounts of exogenous carbon will cause changes in the mineralization rate of SOC [16]. For example, small amounts of exogenous carbon may accelerate SOC mineralization [17]. Despite its importance, the effect of varying amounts of WD on SOC is crucial but often neglected [18]. Thus, in this study, we focused on the effect of the amount of FWD on active SOC fractions and soil enzyme activities.
Numerous studies have found that the storage of FWD is affected by factors such as location and forest type, and is usually between 1 and 15 Mg·hm−2 [13,16]. Therefore, we set up four FWD retention experiments, no retention (CK, 0 kg·m−2), low retention (LR, 0.75 kg·m−2), medium retention (MR, 1.5 kg·m−2), and high retention (HR, 3.0 kg·m−2). Due to its high ornamental and ecological value, Picea koraiensis Nakai is widely planted in northeast China [19]. Its abundant woody residues provide the ideal conditions for studying how FWD affects the concentrations of SOC fractions and enzyme activities. We conducted FWD retention experiments in a P. koraiensis plantation in Harbin, Heilongjiang Province. The cold climate and soil properties in northeast China also pose unique challenges to the decomposition process of FWD, which provides a useful comparative perspective for understanding carbon cycling under different climate conditions around the world. The main objectives of this study are as follows: (1) to investigate the effects of different amounts of FWD on the concentrations of SOC fractions and soil enzyme activities and (2) to elucidate the response of SOC stability to varying amounts of retained FWD. This study proposes the following hypotheses: (1) FWD retention will significantly increase the concentrations of SOC fractions and enhance soil enzyme activities by enhancing microbial activity and (2) FWD retention may disturb the stability of the SOC pool, especially by accelerating carbon transformation and decomposition processes, thus reducing the stability of SOC pool. Our findings are anticipated to guide FWD management strategies in the urban forests of northeast China, aiming to optimize carbon sequestration and enhance ecosystem services. Furthermore, future research should extend this framework to evaluate the long-term effects of FWD under diverse climatic conditions, particularly in regions with contrasting temperature and precipitation regimes.

2. Materials and Methods

2.1. Sample Plot Overview and Experimental Design

This study was conducted at the Urban Forestry Demonstration Base in Harbin City, Heilongjiang Province, China (126°37′45″ E, 45°43′10″ N). The Harbin Urban Forestry Demonstration Base was established in 1948. It is located on the Northeast Forestry University campus, near the Majiagou River, residential areas, and major traffic routes in Harbin. The base spans approximately 44 hectares, with around 63% of forested area. The terrain is flat with an average elevation of about 142 m above sea level. The climate is cold, with an annual mean temperature of 3.6 °C. The lowest temperature was recorded in January (−38.1 °C) and the highest in July (36.4 °C). The mean annual precipitation is about 600 mm. The soil at the study sites is natural soil, known as zonal chernozem, with a gentle terrain and good moisture conditions [20,21].
For this study, we selected urban P. koraiensis plantations at the Urban Forestry Demonstration Base, which were planted in 1952 with 5-year-old P. koraiensis at a current density of 650 plants ha−1 (Figure 1a). In March 2022, we removed all CWD from the site, shredded the remaining FWD into pieces measuring 1–5 cm in length and 0.5–2 cm in diameter, and piled them under the forest. Following previous studies [22,23], on 15 and 16 April 2022, we set the treatments for the effect of FWD retention on the soil of the urban P. koraiensis plantations as CK, LR, MR, and HR. The experiments followed a completely randomized block design with three replicates for each treatment, and a total of 12 sample plots of 2 × 2 m2 were established (Figure 1b). The FWD was placed directly on the soil surface without disturbing the top soil layer of the forest.

2.2. Sampling and Analysis

On 16 and 18 May 2023, we took four random cores from the 0–10 cm and 10–20 cm thick soil layers within the humus horizon of each plot, excluding the forest floor and the subsoil beneath. The soil was characterized by a dark color and high organic matter content. We then prepared two composite samples by pooling the individual samples from the same soil layer. We sieved the fresh samples (<2 mm) to remove roots and coarse stone fragments and then divided each sample into two subsamples. One subsample was stored at 4 °C for the analysis of DOC, MBC, and enzyme activities, while the other subsample was air-dried and used for the analysis of basic soil parameters such as SOC, total nitrogen (TN), and soil pH.
We used the cutting ring method to sample and determine the soil bulk density (SBD, g/cm3) and soil moisture content (SMC, %). Under the condition of a soil to water ratio of 1:2.5, the soil pH was determined using a PHS-3C (Leici Instrument Factory, Shanghai, China). The ring blades were dried in an oven at 105 °C for 24 h. SBD and SMC were measured by the change in weight before and after drying. We analyzed the contents of SOC and TN using a Vario Macro carbon and nitrogen analyzer (Elementar, Frankfur, Germany). We extracted NH4+-N and NO3−-N in the soil with a 2 mol·L−1 potassium chloride solution and determined them with a flow analyzer (BRAN+LUEBBE-AA3, Seal Analytical, Norderstedt, Germany). After the digestion of the soil with concentrated sulfuric acid and perchloric acid, we determined the total phosphorus (TP) in the soil with a flow analyzer (BRAN+LUEBBE-AA3, Seal Analytical, Norderstedt, Germany) and the total potassium (TK) in the soil with a flame photometer. EOC was determined using the KMnO4 (333 mmol·L−1) oxidation method [8]. DOC was obtained by the successive extraction of soil samples with distilled water. We calculated the MBC by multiplying the difference between the fumigated and unfumigated samples by a conversion factor of 2.22 [24]. We determined the C concentration in the extracts using a Multi N/C 2100 analyzer (Analytik Jena, Jena, Germany). POC was separated using the sodium hexametaphosphate leaching method, and the carbon content was measured with the HT1300 analyzer (Analytik Jena, Jena, Germany) [25]. Referring to previous studies, we express (SOC-POC)/SOC as the stability of the SOC pool, with a higher ratio indicating a stronger stability of the SOC pool [26].
According to the method of Qi [27], we measured the activities of cellobiohydrolases (CBHs), β-1,4-glucosidases (βGs), β-xylosidase (βX), leucine aminopeptidase (LAP), and polyphenol oxidase (PPO) because these enzymes play key roles in the breakdown of organic matter, including cellulose and lignin, which are critical in SOC turnover and carbon cycling [28]. Soil enzyme activities were measured using a microplate reader (BioTek, Winooski, VT, USA) (nmol·g−1·h−1).

2.3. Statistical Analyses

One-way analysis of variance combined with Duncan’s post hoc test (p < 0.05) was used to assess the significance of different FWD retention levels on the SOC fractions and enzyme activities in the urban P. koraiensis plantations. All data are expressed as the mean ± standard deviation of three replicates. The statistical analysis was performed with IBM SPSS Statistics 19 (IBM Corp., Armonk, NY, USA). Random forest modeling was conducted using the “random forest” package in R (R Core Team, Vienna, Austria, version 4.3.2) to identify the key variables influencing the stability of SOC pool changes under the FWD retention treatments. We employed Pearson and Mantel correlation analyses to examine the relationships between the SOC fractions and enzyme activities in the FWD within the urban plantations.

3. Results

3.1. Physicochemical Properties of Soil

FWD did not cause significant changes in SBD, SMC, pH, SOC, TP, or NH4+-N (p > 0.05). Compared with CK, in the 0–10 cm soil layer, the MR treatment significantly reduced TN; the HR treatment significantly reduced TK; and the LR, MR, and HR treatments significantly reduced NO3−-N (p < 0.05). In the 10–20 cm soil layer, the HR treatment significantly reduced TK (p < 0.05) (Table 1).

3.2. Active Soil Organic Carbon Fractions

In the 0–10 cm soil layer, the DOC concentration in the LC treatment was significantly higher than that in the CK, MR, and HR treatments. The concentration of MBC was significantly higher in the HR treatment than in the LR and MR treatments. The EOC concentration of the HR treatment was significantly higher than that in the CK, LR, and MR treatments. The POC concentration in the LC and HC treatments was significantly higher than that in the CK and MR treatments (p < 0.05). In the 10–20 cm soil layer, the DOC content in the LR and HR treatments was significantly higher than that in the CK treatment. The MBC concentration was significantly higher in the HR treatment than in the CK and MR treatments. The EOC concentration of the MR treatment was significantly higher than that in the CK treatment. The POC concentration was significantly higher in the MR and HR treatments than that in the CK and LR treatments (p < 0.05) (Figure 2).

3.3. Soil Enzyme Activities

The soil enzyme activities changed significantly with the amount of FWD retained (Figure 3). The LR and MR treatments significantly increased the C-degrading enzymes (CBH, βG, and βX) in the 0–10 cm soil layer and the CBH and βG enzyme activities in the 10–20 cm soil layer (p < 0.05). The HR treatment significantly increased the CBH and βX enzyme activities in the 0–10 cm soil layer and significantly increased the βG enzyme activity in the 10–20 cm soil layer (p < 0.05). The LR and MR treatments significantly reduced the LAP enzyme activity in the 0–10 cm soil layer (p < 0.05). The FWD retention treatments did not cause a significant change in PPO enzyme activity (p > 0.05).

3.4. The Stability of Soil Organic Carbon Pool

The stability of the SOC pool changed significantly with the amount of FWD retained (Figure 4a). The LR and HR treatments significantly reduced the stability of the SOC pool in the 0–10 cm soil layer (p < 0.05). The MR and HR treatments significantly reduced the stability of the SOC pool in the 10–20 cm soil layer (p < 0.05). The random forest model evaluated all the indicators measured in this experiment to identify the key factors driving changes in the stability of the SOC pool under the various FWD retention treatments (Figure 4b). The random forest model identified FWD (21.02%), CBH (19.92%), TK (19.69%), and βX (17.7%) as the variables significantly affecting the stability of the SOC pool (p < 0.05).
The Mantel analysis revealed significant relationships between the physicochemical properties of the soil and the soil enzyme activities (Figure 5). SBD, SOC, DOC, and EOC had significant effects on C-degrading enzymes (p < 0.01). FWD, SMC, NO3−, SOC, TN, TP, and MBC had significant effects on N-degrading enzymes (p < 0.05). The physicochemical properties of soil had no significant effect on PPO enzyme activities (p > 0.05).
FWD retention was significantly negatively correlated with TK and the stability of the SOC pool, but significantly positively correlated with MBC and POC (p < 0.05). SMC was significantly positively correlated with NH4+, NO3−, SOC, TN, TK, and EOC (p < 0.05), while SBD was significantly negatively correlated with most nutrient indicators (p < 0.05). SOC, TN, and TP were significantly positively correlated with DOC, MBC, and EOC (p < 0.05) (Figure 5).

4. Discussion

4.1. Changes in Active SOC Fractions Caused by Retained FWD

In our study, we did not find that retaining FWD had any effect on the basic properties of the soil and SOC concentrations (Table 1). This result has also been supported by many studies [6,29,30]. The decomposition of WD after entering the soil is a long process, which is divided into the following two stages: the initial stage is a rapid decomposition stage, in which microorganisms mainly use active organic matter such as protein, soluble organic matter, and cellulose as C sources, and the mineralization speed is relatively fast; then, it enters a slow decomposition stage, in which microorganisms mainly use N elements and relatively stable organic matter in the soil, and through a complex transformation process, transform wood residues into stable organic matter, thereby increasing the content and turnover rate of SOC [31]. However, the specific extent of WD’s impact on SOC is often restricted by a variety of factors. For example, Fekete et al. conducted an 8-year forest experiment with a double WD treatment and found that the double WD treatment increased the shallow SOC content compared with the control, but not significantly [30]. Previous studies have also shown that the addition of exogenous carbon to soils with different masses leads to different changes in SOC concentrations [32]. This study investigated the basic properties of soil and SOC content 15 months after FWD was retained. The short decomposition time may have been the main reason why the basic properties of the soil and SOC concentrations did not change [25]. In addition, the rapid mineralization of SOC is also one of the reasons. WD serves as a significant carbon source with a high C:N ratio for microorganisms. However, this can temporarily reduce soil nitrogen availability and disrupt the C:N ratio balance. In such scenarios, microbial access to nitrogen becomes constrained, necessitating the mineralization of soil organic matter to acquire nitrogen for microbial growth and reproduction [33]. This process links nitrogen and carbon mineralization, where nitrogen mineralization occurs alongside carbon decomposition, resulting in rapid changes in soil organic matter turnover—a phenomenon known as the priming effect [34]. In addition, the NO3− concentration in the FWD treatment was significantly lower than that in the CK treatment, further demonstrating that the microbial mineralization of SOC was necessary to meet nitrogen demands [35].
Since active SOC fractions can regulate the organic matter and nutrient availability in soil and are highly sensitive to environmental changes, active organic carbon can be used as an early indicator of soil quality [36]. Currently, most studies indicate that the input of exogenous carbon can change the concentrations of active SOC fractions [5]. Active SOC fractions that actively contribute to soil carbon have a significant impact on the stability of the SOC pool [23] and respond rapidly to changes in the soil environment [37]. The organic matter produced during the decomposition of WD is fixed in the soil, especially in our study, where the decomposition of WD was accelerated by shredding [38]. In our study, we found that the LR and HR treatments had a more significant effect on improving the active SOC fractions (Figure 1). However, a large number of studies show that priming intensity is positively correlated with the amount of exogenous carbon supplied [39]. Nevertheless, some studies have indicated that the priming effect decreases when the supply of exogenous carbon exceeds 50% of the MBC, and when it far exceeds the MBC, the priming effect approaches zero or is even negative [34]. Therefore, MR may lead to a “carbon starvation” state because it fails to significantly enhance microbial metabolism and may inhibit the accumulation of active SOC fractions [40].
Correlation analysis showed that FWD retention was significantly positively correlated with MBC and POC (Figure 5). WD can serve as a substrate for microbial decomposition and can provide a stable carbon supply, especially in the form of lignin and cellulose, which are metabolized into microbial biomass and incorporated into the SOC pool, ultimately changing the MBC [41]. Some studies have pointed out that when the availability of nitrogen in soil cannot meet the needs of microorganisms, microbial communities may prefer to obtain nitrogen in decomposition, which may lead to the accumulation of POC [33]. These findings indicate that maintaining the quality and physical and chemical properties of soil affected by WD can alter SOC fractions. In contrast to the majority of studies, our research found no significant impact on the physical and chemical properties of the soil (p > 0.05) (Table 1) [42]. This discrepancy could be due to the frequent rainfall at the sampling site prior to data collection, which minimized the variability of the soil properties [43].

4.2. Changes in Soil Enzyme Activities Caused by Retained FWD

The release of reactive SOC fractions may stimulate enzyme activities and alter the composition of soil decomposer communities [37]. Previous studies have shown that FWD provides a substantial carbon source for soil microorganisms, significantly enhancing the activities of enzymes secreted by them [44]. Our results showed that the LR treatment significantly increased the activities of CBH, βG, and βX in both the 0–10 cm and 10–20 cm soil layers compared to CK (p < 0.05) (Figure 3). These findings suggest that even minimal inputs of FWD can stimulate microbial activity, promote the production and release of carbon-degrading enzymes, and enhance overall enzyme activities, thereby promoting carbon turnover [45]. These enhanced enzyme activities are likely responsible for the observed shifts in SOC fractions, especially the strong correlation between DOC and C-degrading enzymes (Figure 5). This highlights that FWD input not only enhances soil carbon cycling by directly supplying carbon, but also accelerates the decomposition and transformation of organic matter by stimulating microbial activity [46].
The activity of LAP was significantly lower in the LR and MR treatments compared to the CK and HR treatments. Nutrient availability and substrate accessibility are crucial factors affecting enzyme activities. According to resource allocation theory, soil microorganisms can enhance the availability of limited nutrients by adjusting enzyme secretion, thereby mitigating nutrient deficiencies [47]. At present, it is considered that cellulolytic fungi (e.g., Ascomycota) and copiotrophic bacteria (e.g., Proteobacteria) are key drivers of C-degrading enzymes like CBH and βG, particularly under an elevated carbon availability [48]. Our observed increase in C-degrading enzymes may reflect a proliferation of these taxa in response to FWD-derived cellulose inputs (Figure 3). In this study, while WD addition provided cellulose as a key carbon source, the nitrogen levels remained insufficient. Consequently, microorganisms adapted their enzyme production to the fluctuating nutrient conditions, balancing microbial needs with the environmental nutrient availability [49]. This situation may be more conducive to the survival of fungal communities, as fungi exhibit a higher tolerance for C:N ratios and possess the ability to decompose lignin [14]. This could be one of the reasons for the decrease in LAP activity, as fungi prioritize the production of PPO [50]. When easily degradable carbon sources become scarce, microorganisms are known to shift towards more resistant carbon sources [51]. Studies have shown that, when the readily available carbon in the soil increases, microorganisms do not need to decompose the stubborn carbon, so the PPO enzyme activity decreases [52]. This is consistent with the results of our study, where the active SOC fraction increased and the PPO enzyme activity decreased (Figure 2 and Figure 3). Microorganisms utilize unstable organic carbon from FWD by secreting carbon-degrading enzymes, thereby improving the turnover efficiency of EOC and DOC [50]. These results suggest that retaining FWD enhances soil C-degrading enzyme activities, improving the microbial utilization of FWD. Our findings partially support our hypothesis that retaining FWD increases the concentrations of active SOC fractions and C-degrading enzyme activities while reducing the concentrations of the activities of LAP and PPO.

4.3. The Stability of Soil Organic Carbon Pool Was Changed After Retaining FWD

Our study demonstrates that retaining FWD significantly altered and reduced the stability of the SOC pool (Figure 3). The high C:N ratio of FWD plays a critical role in determining the effectiveness of exogenous carbon for SOC stability [53]. This input of carbon is not uniformly beneficial for soil carbon retention; rather, it can lead to the destabilization of the SOC pool by triggering microbial processes that favor carbon mineralization [35]. Microbial stoichiometric balance, which involves maintaining a steady ratio of carbon to nutrients like nitrogen, is essential for effective decomposition and carbon cycling. FWD, with its high C:N ratio, can induce microbial nitrogen deficiency, which is critical for microbial decomposition pathways. This deficiency accelerates the priming effect and reduces the stability of the SOC pool [33]. Consequently, a balanced organic matter turnover is essential for sustainable soil management and effective carbon sequestration.
Our results suggest that FWD retention significantly increased the concentration of POC. This indicates that, while FWD contributes to short-term carbon inputs, it may reduce the long-term storage capacity of SOC. These findings have important implications for carbon sequestration in urban forests, where inputs of woody debris may destabilize existing carbon pools rather than enhancing them [54]. Additionally, nitrogen limitation may restrict the transformation of POC into more stable SOC, further reducing the long-term storage capacity of SOC [4].
The FWD retention quantity and POC, CBH, and βX enzyme activities are the factors that significantly affect the stability of the SOC pool (Figure 4), while the FWD retention quantity, POC, and the stability of the SOC pool are negatively correlated (Figure 5) [54]. Increased enzyme activities, driven by the input of easily available carbon sources like FWD, accelerate the decomposition of SOC, making it more susceptible to loss. These findings align with previous studies that have demonstrated how elevated carbon inputs can reduce SOC stability by stimulating microbial activity [49]. Our findings indicate that FWD retention substantially influences the turnover and stability of the SOC pool, primarily by altering microbial dynamics and enzyme activities rather than through direct changes in soil physical and chemical properties. In conclusion, our study demonstrates that, while retaining FWD can enhance the availability of active SOC components, it also accelerates the decomposition of SOC, thereby reducing the stability of the SOC pool. This supports our second hypothesis that FWD retention disturbs the stability of the SOC pool. These findings suggest that urban forest management strategies should carefully consider the balance between short-term carbon inputs and long-term SOC stability.

5. Conclusions

This study demonstrates that retaining FWD significantly impacts the SOC fractions and enzyme activities in urban P. koraiensis plantations in northeast China. Our findings indicate that FWD retention increases the concentration of active organic carbon components and the activity of C-degrading enzymes, while decreasing the concentration of the activities of LAP and PPO enzymes. These results suggest that a more in-depth understanding of the long-term effects of FWD retention on the SOC pool is needed. In summary, maintaining a considerable amount of FWD in urban forests in northeast China can effectively increase the concentration of organic carbon components in active soil, which is beneficial for the soil carbon cycle of urban plantations. However, it is important to consider that this practice may also reduce the long-term stability of the SOC pool. Future research should focus on investigating the long-term effects of WD on soil microbial communities and other ecological processes to gain a more comprehensive understanding of the ecological impacts of this management practice.

Author Contributions

Conceptualization, L.Y.; methodology, H.X.; software, H.Z.; validation, L.Y., H.X. and H.Z.; formal analysis, H.X.; investigation, H.Z.; resources, H.X. and H.Z.; data curation, H.X.; writing—original draft preparation, H.X.; writing—review and editing, L.Y.; visualization, H.X.; supervision, H.Z.; project administration, L.Y.; funding acquisition, L.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key R&D Projects of China, grant number 2024YFD2200202.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.

Conflicts of Interest

The authors declare that there are no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
WDWoody Debris
FWDFine Woody Debris
CWDCoarse Woody Debris
SOCSoil Organic Carbon
MBCMicrobial Carbon
EOCEasily Oxidizable Organic Carbon
DOCDissolved Organic Carbon
POCParticulate Organic Carbon
SBDSoil Bulk Density
SMCSoil Moisture Content
CBHCellobiohydrolases
βGβ-1,4-glucosidases
βXβ-xylosidase
LAPLeucine aminopeptidase
PPOPolyphenol oxidase

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Figure 1. Location of the Forestry Demonstration Base and P. koraiensis plantations (a) and layout of the randomized block design with four FWD retention treatments (b).
Figure 1. Location of the Forestry Demonstration Base and P. koraiensis plantations (a) and layout of the randomized block design with four FWD retention treatments (b).
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Figure 2. Dissolved organic carbon (a), microbial carbon concentration (b), easily oxidizable organic carbon concentration (c), and particulate organic carbon (d) in urban P. koraiensis plantations with retained FWD. Note: Data are presented as mean ± standard error (n = 3). Different letters mean that there are significant differences between different treatments in the same soil layer (p < 0.05).
Figure 2. Dissolved organic carbon (a), microbial carbon concentration (b), easily oxidizable organic carbon concentration (c), and particulate organic carbon (d) in urban P. koraiensis plantations with retained FWD. Note: Data are presented as mean ± standard error (n = 3). Different letters mean that there are significant differences between different treatments in the same soil layer (p < 0.05).
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Figure 3. Cellobiohydrolases enzyme activity (CBHs, (a)), β-1,4-glucosidases enzyme activity (βGs, (b)), β-xylosidase enzyme activity (βX, (c)), leucine aminopeptidase enzyme activity (LAP, (d)), and polyphenol oxidase enzyme activity (PPO, (e)) in urban P. koraiensis plantations with retained FWD. Note: Data are presented as mean ± standard error (n = 3). Different letters mean that there are significant differences between different treatments in the same soil layer (p < 0.05).
Figure 3. Cellobiohydrolases enzyme activity (CBHs, (a)), β-1,4-glucosidases enzyme activity (βGs, (b)), β-xylosidase enzyme activity (βX, (c)), leucine aminopeptidase enzyme activity (LAP, (d)), and polyphenol oxidase enzyme activity (PPO, (e)) in urban P. koraiensis plantations with retained FWD. Note: Data are presented as mean ± standard error (n = 3). Different letters mean that there are significant differences between different treatments in the same soil layer (p < 0.05).
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Figure 4. The stability of soil organic carbon pool (a) and random forest analysis of environmental factors affecting the stability of soil organic carbon pool (b) in urban P. koraiensis plantations with retained FWD. Note: Data are presented as mean ± standard error (n = 3). Different letters mean that there are significant differences between different treatments in the same soil layer (p < 0.05). The asterisk (*) indicates statistical significance (* p < 0.05).
Figure 4. The stability of soil organic carbon pool (a) and random forest analysis of environmental factors affecting the stability of soil organic carbon pool (b) in urban P. koraiensis plantations with retained FWD. Note: Data are presented as mean ± standard error (n = 3). Different letters mean that there are significant differences between different treatments in the same soil layer (p < 0.05). The asterisk (*) indicates statistical significance (* p < 0.05).
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Figure 5. Mantel test between soil enzyme and soil factors, as well as Pearson correlation coefficients within soil variables in urban P. koraiensis plantations with retained FWD. Note: The asterisk (*) indicates statistical significance (* p < 0.05).
Figure 5. Mantel test between soil enzyme and soil factors, as well as Pearson correlation coefficients within soil variables in urban P. koraiensis plantations with retained FWD. Note: The asterisk (*) indicates statistical significance (* p < 0.05).
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Table 1. Physicochemical properties of soil in urban P. koraiensis plantations with retention of FWD.
Table 1. Physicochemical properties of soil in urban P. koraiensis plantations with retention of FWD.
Layer
(cm)		TreatmentSBD
(g·cm−3)		SMC
(%)		pHSOC
(g·kg−1)		TN
(g·kg−1)		TP
(mg·kg−1)		TK
(mg·kg−1)		NH4+-N
(mg·kg−1)		NO3−-N
(mg·kg−1)
0–10CK0.93 ± 0.10 A28.24 ± 4.36 A5.70 ± 0.10 A40.97 ± 4.15 A3.12 ± 0.26 A733.47 ± 100.11 A5.59 ± 0.25 AB1.04 ± 0.38 A45.26 ± 1.85 A
LR0.9 A ± 0.02 A26.06 ± 1.35 A5.53 ± 0.07 A42.41 ± 2.20 A3.22 ± 0.66 A767.86 ± 58.97 A5.85 ± 0.18 A1.26 ± 0.02 A21.68 ± 2.28 C
MR1.08 ± 0.03 A25.39 ± 2.43 A5.70 ± 0.21 A37.30 ± 3.75 A2.19 ± 0.20 B707.53 ± 35.54 A5.12 ± 0.18 BC0.92 ± 0.17 A15.03 ± 0.26 D
HR0.94 ± 0.11 A26.93 ± 5.83 A5.83 ± 0.15 A44.94 ± 2.92 A3.71 ± 0.53 A809.91 ± 81.83 A4.69 ± 0.10 C0.97 ± 0.33 A27.85 ± 0.68 B
10–20CK1.29 ± 0.11 A21.82 ± 0.34 A5.93 ± 0.03 A28.81 ± 1.09 A1.83 ± 0.37 A546.54 ± 31.83 A5.58 ± 0.14 A0.59 ± 0.05 A29.05 ± 8.10 A
LR1.10 ± 0.06 A22.68 ± 1.32 A5.73 ± 0.17 A29.12 ± 1.01 A1.92 ± 0.13 A645.49 ± 99.13 A5.86 ± 0.08 A1.32 ± 0.65 A14.51 ± 3.37 A
MR1.09 ± 0.03 A21.90 ± 1.07 A5.63 ± 0.09 A28.78 ± 2.83 A1.97 ± 0.38 A570.46 ± 30.95 A5.27 ± 0.47 AB0.67 ± 0.05 A14.90 ± 1.87 A
HR1.16 ± 0.14 A21.6 ± 1.18 AA5.80 ± 0.15 A27.24 ± 1.74 A1.86 ± 0.46 A577.90 ± 81.56 A4.49 ± 0.14 B0.70 ± 0.06 A22.13 ± 2.73 A
Note: Data are presented as mean ± standard error (n = 3). Different letters mean that there are significant differences between different treatments in the same soil layer (p < 0.05).
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Xing, H.; Zhang, H.; Yang, L. Retention of Fine Woody Debris Reduces Stability of Soil Organic Carbon Pool by Changing Soil Organic Carbon Fractions and Enzyme Activities in Urban Picea koraiensis Plantations. Forests 2025, 16, 434. https://doi.org/10.3390/f16030434

AMA Style

Xing H, Zhang H, Yang L. Retention of Fine Woody Debris Reduces Stability of Soil Organic Carbon Pool by Changing Soil Organic Carbon Fractions and Enzyme Activities in Urban Picea koraiensis Plantations. Forests. 2025; 16(3):434. https://doi.org/10.3390/f16030434

Chicago/Turabian Style

Xing, Honglin, Hao Zhang, and Ling Yang. 2025. "Retention of Fine Woody Debris Reduces Stability of Soil Organic Carbon Pool by Changing Soil Organic Carbon Fractions and Enzyme Activities in Urban Picea koraiensis Plantations" Forests 16, no. 3: 434. https://doi.org/10.3390/f16030434

APA Style

Xing, H., Zhang, H., & Yang, L. (2025). Retention of Fine Woody Debris Reduces Stability of Soil Organic Carbon Pool by Changing Soil Organic Carbon Fractions and Enzyme Activities in Urban Picea koraiensis Plantations. Forests, 16(3), 434. https://doi.org/10.3390/f16030434

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